Place oil in a dutch oven (preferably cast iron) and heat over medium-high heat for 1 minute. Add onion and cook for 4 minutes or until translucent. Add remaining ingredients except lime juice and garnishes. Bring to a boil, reduce heat, and simmer, uncovered, for 20 minutes. Remove from heat.

One cup at a time, transfer soup to a blender and blend until smooth. Return puree to dutch oven and reheat over medium heat for about 5 minutes. Stir in lime juice immediately and serve with tortilla chips topped with a dollop of sour cream, scallion, chopped tomatoes, and cheddar cheese.